1. Field of the Invention
The present invention relates to radar tracking systems, and, more specifically, to systems which correct for eclipsing by transmitted pulses and/or cluttering of the Doppler frequency interval.
2. Description of the Related Art
To detect and track a target, Doppler radars must consider two problems when selecting a PRF. One is the signal-to-noise ratio (SNR) and the other is the signal-to-clutter ratio (SCR) it can achieve on the target of interest. A high duty factor with little or no eclipsing may be required to achieve an adequate SNR while the target's Doppler frequency should not overlap the Doppler frequency of the clutter to obtain the highest SCR.
High PRF radars use PRFs sufficiently high so the target return is not masked by clutter returns to ensure a high SCR. This PRF must be greater than the full Doppler clutter bandwidth (4*Vr/.lambda.) plus the maximum target Doppler frequency of interest (2*Vt/.lambda.) where Vr is the radar's velocity, Vt is the target's velocity along the line of sight to the radar and .lambda. is the transmission wavelength of the radar. Range eclipsing is very severe when high PRFs are used since the transmitted pulse occurs at ranges of n*c/2*PRF where c is the velocity of light and n is an integer from 0 to infinity. Thus eclipsing occurs at each of these ranges over the interval covered by the transmitted pulse (c*.tau./2) where .tau. is the time duration of the transmitted pulse which may result in an inadequate SNR.
Low PRF radars use sufficiently low PRFs such that the range between pulses n*c/2*PRF greater than the greatest target range of interest. Then eclipsing of interest only occurs for ranges less than the short ranges covered by the pulsewidth (c*.tau./2). However, the clutter return now occurs around every PRF line (n*PRF) and the target return is quite likely to be masked by the clutter return yielding a poor SCR.
Thus medium PRF systems are often used to provide a compromise between these extremes. Current medium PRF radar systems use several PRFs to ensure seeing a target over a wide range interval and/or Doppler frequency interval (to avoid significant eclipsing of the target return signal by the transmitted pulse in the time domain and to avoid having clutter mask the desired target return in the frequency domain). They may process the returns from each PRF independently or integrate the returns of the different PRFs. This patent presents a method of rapidly and automatically selecting a PRF to yield no eclipsing and no clutter masking given the target range and target Doppler frequency are approximately known and given such a solution is realizable. This algorithm therefore permits higher target update rates since multiple PRFs need not be processed.